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NATIONAL INSTITUTE OF STANDARDS & 

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'<■ cy\j o 



ro 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



/ Vv 



Bureau of Standards 



S. W. STRATTON. Director 



(Prior to Volume 15 this series was called the *' Bulletin 
of the Bureau of Standards") 



Volume 17, part 2 

1922 




WASHINGTON 

GOVERNMENT PRINTING OFTICE 

1922 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON. Director 



No. 437 

THE SOLUBILITY OF DEXTROSE 
IN WATER 

BY 

RICHARD F. JACKSON, Associate Chemist 
CLARA GILLIS SILSBEE, Assistant Chemist 

Bureau of Standards 



MAY 5, 1923 




PRICB, S CENTS 

Sold only by the Superintendent of Documents, Government Printins OflSce 
Washington, D. C. 



WASHINGTON 
GOVERNMENT PRINTING OFFICE 

1922 



THE SOLUBILITY OF DEXTROSE IN WATER 

By Richard F. Jackson and Clara Gillis Silsbee 



ABSTRACT 

The solubilities of dextrose, in addition to their scientific interest, have become of 
fundamental importance in controlling the processes of manufacture. They have 
been determined over a range of temperatures extending to go° C. Three solid phases 
are capable of existence, namely, ice, a-dextrose monohydrate, and anhydrous a-dex- 
trose. From the freezing point curve, computed from existing data, and the satura- 
tion curve of dextrose hydrate, the cryohydric point was found. Dextrose hydrate 
is stable between — s.°3 C and 50° C; its solubility has a high temperature coefficient. 

The transition from dextrose hydrate to anhydrous dextrose is shown to occur at 
50° C. Above this temperature anhydrous dextrose is the solid phase. Its tempera- 
ture coefficient is relatively small. Below the transition temperature the anhydrotis 
form may persist in unstable equilibrium, even at a temperature as low as 28° C. 



CONTENTS 

Page. 

1. Introductory 715 

2 . Historical 716 

3. General description of solubility measurements 716 

4. The solid phase; ice 718 

5. The cryohydric point 719 

6. The solid phase; a-CgHjjOg.HjO 719 

7. The melting point of dextrose hydrate 721 

8. The transition point 723 

9. The solid phase; a-CgHjjOg 723 

10. Summary 724 

1. INTRODUCTORY 

In very recent times it has been found commercially feasible to 
crystallize dextrose from water solution and to separate the crys- 
tals from the mother liquor by centrifugal machines in much the 
same manner as the corresponding step in cane sugar manufacture 
is carried out. A similar process ^ was attempted about forty 
years ago, but the effort resulted in failure. With the advent of 
modem machinery and more scientific methods, however, the 

' Making crystalline dejctrose, C. £. G. Forst, Stlgar, 23, p. 3S0; igai. I/misiana Planter, 67, p. 14; 
July 2, 1921. 



7i6 Scientific Papers of the Bureau of Standards [Voi.17 

industry is at present being revived on a large scale with every 
prospect of success. 

As a fundamental basis for calculating supersaturation coeffi- 
cients, for estimating crystallizer performance, and the more 
extended investigation of the effect of nonsugars in the process of 
crystallization, the solubility of the pure substance in water 
assumes a technical as well as scientific importance. To serve 
these piurposes we have imdertaken the determination of solubilities 
over a wide range of temperatures. 

2. HISTORICAL 

A survey of the hterature revealed but one solubility measure- 
ment, and that at only one temperature. Anthon ^ in 1883 found 
that at 15° C 100 parts of water dissolved 81.68 parts of anhydrous 
dextrose, giving a solution of 44.96 per cent. Aside from this, 
no data are in existence. Maquenne,^ however, states that at 
100° C dextrose and water are miscible in all proportions. 

3. GENERAL DESCRIPTION OF SOLUBILITY MEASUREMENTS 

The solubility measurements were made by agitating the mix- 
ture of dextrose and water with a large proportion of the solid 
phase in a rotating frame under the water of an electrically 
operated thermostat. After equilibrium had been reached, the 
solution was separated from the crystals and analyzed. The 
separation of the finely divided crystals from the viscous saturated 
solution was accomplished by filtration under pressure through 
an asbestos mat, the arrangement of which is shown diagram- 
matically in Fig. i . 

The filtration tube A was constructed of glass tubing 20 mm 
inside diameter with walls 3 mm thick. In this was inserted the 
perforated brass disc B which was edged with lead and covered 
with an asbestos mat. For temperatures much removed from that 
of the laboratory, the transference from the solubility bottle C 
was accomplished as shown in position /. The solubility bottle 
was removed from the rotating frame with a pair of tongs and its 
mouth held above the liquid of the thermostat and wiped dry. 
A rubber stopper was inserted which carried the wide bore glass 
tube D, which extended nearly to the bottom of the bottle. The 
glass tube passed through another stopper into the filtration tube, 

' V. lyippinann, Die chemie der zuckerarten, 1, p. 266; 1904. 
' I/BS sucres et principaux derives, p. 479; ipcxj. 



Jackson"] 
Silsbee J 



Solubility of Dextrose in Water 



717 



^^ 



B 




B 



-J 



Fig. I. — Apparatus for filtration of viscous solutions under pressure 
93045°— 22 2 



7i8 Scientific Papers of the Bureau of Standards [Voi.i? 

the latter being arranged with respect to the remaining parts oi 
the apparatus as shown in position //, with the exception of the 
valve G and brass plate H. The whole apparatus, with the excep- 
tion of the upper parts of the two small glass tubes E and F, was 
then irmnersed in the water of the thermostat and allowed to take 
its temperature. A slight air pressure transmitted through the 
tube E sufficed to drive the crystal mixture into the filter tube 
without even a momentary change in temperature. 

After the transference the upper edge of the filter tube A was 
raised above the surface of the liquid of the thermostat and wiped 
dry. The valve G and stopper were inserted in the filter tube and 
held tightly by the brass plate H, which was held in place by the 
three brass posts I. An ordinary bicycle pump suppHed sufficient 
pressure to force the saturated solution through the asbestos filter 
into the weighed container /, which was finally dried, brought to 
the temperature of the balance case, and weighed. 

The weighed sample was transferred to a weighed loo cc volu- 
metric flask, made to volume at 20° C and weighed. The solution 
was allowed to stand over night in order to complete the mutaro- 
tation which occurs to some extent upon dilution of a concentrated 
syrup, and was polarized in a Bates-Fri5 saccharimeter at 20° C. 
By this procedure we obtained a densimetric and polariscopic 
determination of the dextrose in the sample. An agreement by 
these two methods was assumed to be an indication of the accuracy 
of the analytical work. 

For the normal weight of dextrose the value 32.231 g, as pre- 
viously determined by one of us,* was used. The densities of 
dextrose solutions which were determined incidentally to this same 
investigation were used as standards. All analytical results in 
this paper are expressed in terms of anhydrous dextrose. 

4. THE SOLID PHASE; ICE 

Starting with the freezing point of pure water we may plot the 
freezing point curve of dextrose solutions by using the data of 
Roth * for the more dilute solutions, and of Abegg ® for the more 
concentrated solutions. Abegg's determinations are expressed in 
terms of volume concentrations at the freezing point temperatures, 
with no accessory data by which to calculate the concentrations 
by weight. It was consequently necessary to determine the densi- 

< Jackson, B. S. Bulletin, 13, p. 633. 1916 (or B. S. Sci. Papers, No. 293)- 
' Landolt and Bornstein Tabellen, p. 820; 191a. 
«Zeit. Phys. Chem., 15, 222; 1894. 



siifbeT] Solubility of Dextrose in Water 719 

ties of the solutions which possessed the same concentrations per 
volume at the respective temperatures as those concerned in his 
freezing point determinations. Two such solutions were prepared 
and their densities at 0° C and their expansion coefficients were 
accurately determined. From these data their densities and 
volume concentrations were computed for the temperatures at 
which Abegg's freezing point determinations were made. In both 
cases the volume concentrations as thus determined approximated 
Abegg's very closely, and a slight interpolation to his precise con- 
centration was possible without appreciable error. The solution 

/ — 2?30SC\ 

containing 188.02 g per liter at — 2?305 C had a density ( ^^ — I 

of 1.0740 and a weight composition of 17.586 per cent; that con- 
taining 378.0 g per liter at - 5^605 C had a density ( — op — ) o^ 

1. 1 464 and a weight composition of 33.015 per cent. 

The above data are plotted in Fig. 2. The essential agreement 
between Roth's and Abegg's determinations is shown by the 
smoothness of the connecting curve. 

5. THE CRYOHYDRIC POmX 

At the intersection of the freezing point ctu"ve with the extra- 
polated solubility curve of hydrated dextrose occurs the cryo- 
hydric point. The temperature and composition as determined 
graphically proved to be —5^3 C and 31.75 per cent. 

Since no solubility measurements were made below 0° C, the 
possibility is not excluded that a new solid phase may be stable 
between o and —5^3 C. 

6. SOLID PHASE; a-CeHisOg.HjO 

At and above the cryohydric temperattue the stable solid phase 
of the sugar is the monohydrate. The region of stability of this 
phase extends from —5^3 C to exactly 50° C. The solubilities 
are assembled in Table i and are shown graphically in Fig. 2. As 
appears from the diagram, Anthon's solubility measurement at 
15° C is in good agreement with our curve. 

This phase on crystalUzation from water solution in general 
forms minute plates of a lustrous silky appearance. The crystals, 
however, are capable of good development. We are informed by 
Mr. W. B. Newkirk, of the Com Products Refining Co., that when 



720 



Scientific Papers of the Bureau of Standards 



{Voi.ir 



the substance is allowed to crystallize slowly during the process 
of large scale manufacture, crystals of considerable magnitude are 
obtained. Mr. F. P. Phelps, of this Bureau, has succeeded in 
growing perfectly formed crystals 6-7 mm in length. 













































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PERCENT COKCENTRATION 

Fig. 2. — The system, dextrose and water 

The solid curves show the final equilibria with respect to the solid phases, ice, dextrose hydrate, and 
anhydrous dextrose. The dotted curve shows the instaueous solubility before mutarotatiou. AU data 
are expressed in terms of anhydrous dextrose. 

The temperature coefficient of the solubility is very large. 
Thus, if the solubilities in Table i are stated in terms of the parts 
of dextrose dissolved by a constant weight of water, it is seen 
that 100 g of water dissolve, at o?5 C, 54.32; at 30° C, 120.46; 
and at 50° C, 243.76 g of dextrose. 



Jacksonl 
Silsbee J 



Solubility of Dextrose in Water 
TABLE 1.— The Solubility of Dextrose in Pure Water 



721 



Temperature in degrees 
centigrade 


Solid phase 


Dextrose 
in solu- 
tion a 


Temperature in degrees 
centigrade 


Solid phase 


Dextrose 
in solu- 
tion a 


—0.772 1' 


[ice 


Per cent 

f 6.83 
J 16.65 
1 17.59 
I 33. 02 


28.00 


L-CsHiiOe 
(Metastable 


Per cent 

f 66.0 


—2.117 ti.. 


28.00 


67.9 


—2.305 c 


40.00 


1 67.6 


—5.605 c. 


45.00 


69.69 








—5.3 


Cryohydrate 


31.75 


55.22 


•a-CeHisOs 


f 73.08 








+ 0.50 


a-CjHisOs.HjO 


35.2 

44.96 

49.37 

52.99 

54.64 

58.02 

62.13 

62.82 

65.71 


70.2 


{ 78. 23 


15.00 d. 


80.5 


81. 49 


22.98 


90.8 


84.90 


28.07. 






30.00 




35.00.. 




40.40 




41.45 . 




45.00 








50.00 


Transition 


70.91 









a Estimated as anhydrous sugar. 

b Roth, Zeit. Phys. Chem., 43, p. 552; 1903. 

<^ Abegg, Zcit. Phys. Chem., 15, p. 222; 1894. 

<* Anthon. v. Lippmannn, Die Chemie der Zuckerarten, I, p. 266; 1894. 

7. THE MELTING POINT OF DEXTROSE HYDRATE 

The melting point of dextrose hydrate as observed in a capillary 
tube in the usual manner has been found to lie in the vicinity of 
80-90° C, the values reported by different observers ^ being 
strikingly at variance with one another. We have found that if 
the capillary containing the sample is plunged into the bath which 
had previously been brought to about 83° C, incipient fusion 
occurred, although it was found impossible to obtain reproducible 
results. 

At its melting point dextrose hydrate is in equilibrium with a 
solution of its own composition. In other words, the solubility as 
thus determined is 90.9 per cent at 80-85° C. It is at once appar- 
ent from Fig. 2 that by no manner of extrapolation can our 
solubility curve of the hydrate be made to pass through this 
point, and that the discrepancy is so great that some explanation 
is required. 

When crystalline dextrose is dissolved in water, it immediately 
undergoes a partial stereochemical transformation into the 
/3-form at a rate which depends upon the temperature. All of 
the present measurements were made under equilibrium condi- 
tions, the time of agitation of the crystals being continued until 
the reaction, 

a-dextrose ^ct-dextrose ^/3-dextrose 



crystals 



saturated solution 



solution 



' V. Lippmann, Die Chemie der Zuckerarten, I, p. 263; 1904. 



722 Scientific Papers of the Bureau of Standards [Voi.17 

had reached equilibrium. The final solubility is consequently that 
of a-dextrose, not in water, but in a solution of /3-dextrose. On 
the other hand, at the melting point or the temperature of incip- 
ient fusion, the composition of the crystals represents the solu- 
bility of Q!-dextrose in water, since no j3-dextrose can be formed 
until after the crystal has begun to fuse. Even after fusion the 
complete transformation to the equilibrium requires several min- 
utes, during which time the melting point is undergoing rapid 
variation, both because of the formation of the /3-compound and 
because of the rapid efflorescence of the hydrate. With these two 
disturbing effects, it is not stuprising that the melting point should 
fail of reproducibility. It is now of interest to inquire if, on the 
basis of the above discussion, we may reconcile otu solubility curve 
with the observed melting point. If we could plot the solubility 
curve of a-dextrose hydrate in pure water — that is, the instanta- 
neous solubility before mutarotation has begun — such a curve 
should pass through the observed melting point. 

On this curve two points are experimentally realizable. One 
of these is the melting point itself. The other point is the instan- 
taneous solubility at 0° C. The possibility of obtaining experi- 
mental data at 0° C depends upon the fact that the rate of mutaro- 
tation is an important function of temperatvue. Thus the velocity 
constant of mutarotation which, expressed in decimal logarithms 
and minutes, is 0.00662^ at 20? 7 C, becomes about the order of 
o.ooi at 0° C.® The reaction being so slow, it is possible to obtain 
a solubility measurement of the pure a-hydrate in water before 
its transformation has become considerable by violently agitating 
a large excess of solid phase in the presence of its solution for 
short periods of time. The mean of three such determinations 
which were in agreement within 3 per cent indicated that the in- 
stantaneous solubility was 17.5 per cent. 

A concentrated aqueous solution of dextrose after the comple- 
tion of the mutarotation contains the two isomers^" in the ratio of 
about 40 per cent a- and 60 per cent /3-. We may therefore 
from our solubility measurements make an approximate calcu- 
lation of the solubility of the oi-dextrose in pm'e aqueous solution. 
An example will suffice. 

At 20° C the final solubility is 47.5 per cent. One himdred 
grams of the solution then contains 19.0 g oj-dextrose, 28.5 g 

* I^evy, Zeit. Phys. Chem., 17, p. 301; 1895. 

* Nelson and Beegle, J. Amer. Chem. Soc, 41, p. 565; 1919, report 0.00092 at o.°is C. and ph 6.84. 

" Armstrong, The simple carbohydrates and glucosides, p. 17; 1919. Hudson, J. Amer. Chem. Soc, 89, p. 
1018; 1917. 



sihbeT] Solubility of Dextrose in Water 723 

j8-dextrose, and 52.5 g water. We find the solubility of the 
a-form in pure water to be about 26.6 per cent. Proceeding in 
this way we may compute roughly the solubilities of the a- 
dextrose in pure water, as shown in the dotted curve II. This 
curve, which is obviously little more than qualitative in character, 
is seen to be compatible with the observed melting point of 
8c^85° C. 

8. THE TRANSITION POINT 

At 50° C duplicate measurements were made of the solubility 
approached from supersaturation and from undersaturation , re- 
spectively. The solubility proved to be 71.06 per cent. The solid 
phases from which samples of the solution had been taken were 
rapidly purged with aqueous alcohol followed by ether. They 
were then air died and analyzed for their water content. One of 
these was completely anhydrous ; the other contained exactly one 
molecule of water of crystallization. Since these two solid phases 
were in equilibrium with solutions of the same composition at the 
same temperature, it is evident that the temperature of 50° C is 
the "transition point" from the hydrated to the anhydrous crys- 
talline form, in the presence of the solution. It should, however, 
be remarked that this is only a pseudo-transition point. The true 
transition point would be that temperature at which a-dextrose is 
in equilibrium with a solution containing only a-dextrose. The 
solution actually contains about 60 per cent of /3-dextrose, which 
thus lowers the true transition point by a perfectly definite amount. 
The true transition temperature would be of so fugitive a character 
that its measurement not only would involve great experimental 
difficulties, but would be of little practical significance. 

9. THE SOLID PHASE; a-C^B-.^O^ 

At temperatures above 50° C anhydrous dextrose becomes the 
stable solid phase. Its solubiHty rises in very nearly linear rela- 
tion and, compared with that of the hydrate, with a small tem- 
perature coefficient. It forms hard crystals which are capable 
of development to a considerable size. The solubility measure- 
ments are assembled in Table i and plotted in Fig. 2. 

The solubility curve produced to the 100 per cent ordinate 
shows a melting point for the anhydrous form below 135° C, 
whereas the observed melting point is 144-146° C." Here again 

" Maquence, Les sucres et principaux d&ives, p. 478; 1900. 



724 Scientific Papers of the Bureau of Standards [Voi. 17 

the melting point, as indicated by extension of the solubility 
curve, is that of the mixture of a- and jS-dextrose, while the ob- 
served is that of the pure a-dextrose. 

Below the transition temperature it is possible to maintain the 
anhydrous form as the solid phase in a metastable state. We 
have succeeded in extending the solubiUty measurements down 
to 28° C. Our data on this branch of the curve must be considered 
approximate, since the experimental difficulties were considerable. 

In the vicinity of 100° C the anhydrous /S-dextrose becomes the 
stable soUd phase. We have not attempted to determine the 
transformation point or the solubilities. It is possible to predict, 
however, that the solubility curve of the |8-compoimd lies very 
close to the prolongation of the a-, since their observed melting 
points are very close together. The jS-dextrose melts at 148° C, 
the a- at 146° C. 

10. SUMMARY 

The equilibria in the system, dextrose and water have been 
determined. For temperattures below 90° C three solid phases are 
capable of existence, namely, ice, a-dextrose monohydrate, and 
anhydrous a-dextrose. The freezing point curve was computed 
from the data of Roth and of Abegg. The cryohydric point, 
determined graphically, was foimd to lie at the temperatiu-e 
— 5?3 C and concentration 31.75 per cent dextrose. The solid 
phase, a-dextrose monohydrate, which occurs in lustrous plates, 
is stable between — 5?3 C and 50° C. Its solubility shows a very 
high temperature coefficient. Thus, at o?5 C, 100 parts of water 
dissolve 54.32 parts; at 50° C, 243.76 parts of dextrose. The 
observed melting point, 80-90° C, although located far from the 
extrapolated solubility ciuve, is shown to be compatible with the 
measurements, on the theory that ;Q-dextrose is present in the 
saturated solution and absent during a melting point determi- 
nation. Above the transition point, 50° C, the anhydrous form 
becomes stable. Its solubility shows a small temperature coeffi- 
cient. The solubility measurements of this phase in metastable 
state were continued down to 28° C. 

Washington, January 7, 1922. 



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